Logitech X-530. Loud. Clear. Subwoofer.
I'm sure Z220s etc (costing £££s) are another level when you get to really, really, really loud but I am continually astounded at the clarity, volume and bass the X-530 provides, nevermind when you consider the price.
The clarity continues well into "someone standing next to you has to shout" type volume, and even at max volume the dip in sound quality is noticeable but it's not unbearable. Definitely one of the best bits of money spent on my PC. These have been around a while now and something technically better may have come along, but I can't praise the difference these make over any "normal" speakers enough. I paid £40 5 years ago and although these do appear to have gone up in price I'm sure they can be had for less than what ebuyer are asking.
Is it a PC or a laptop? Important question as I think you'll find not many laptops have enough "sockets" to plug in a 5.1 or 7.1 setup whereas even the most basic of circa 2005 computers have minimum 5.1 outputs. Having said that you can use these speakers with a stereo source (e.g. a laptop, PS3 etc) with the included adaptor and a 3.5mm to RCA adaptor (not included) but the subwoofer is a bit weaker. Though presumably the same can be said for a 2.1 setup plugged into a stereo output - regardless, all your music is in stereo anyway.